1. Field of the Invention
This patent specification relates generally to an image forming method and apparatus, and more specifically to such apparatus for use in digital copying, printing machine and facsimile apparatus, provided with improved toner transport capabilities.
2. Discussion of the Background
The electrophotographic image forming process is well known. In image forming apparatuses such as a copying machine, printer and facsimile apparatus, in general, the formation of the images is carried out through the electrophotographic process steps of forming electrostatic latent images on an image bearing member or photoreceptor, developing as visible toner images using toner particles, and transferring the toner images onto a copy sheet which subsequently passes through a fixing unit to form fixed images on the sheet.
When the amount of toner in the fixing unit decreases during the developing steps, the toner is supplied to the fixing unit from a toner container in a copying apparatus, which may be housed separately from one another.
In regard to the toner, some portion thereof remains on the photoreceptor as residual toner following the transfer of toner images during the developing steps. As is well known, this residual portion of the toner is then removed from the photoreceptor by scraping off with a cleaning blade housed in a cleaning unit.
With increasing concern for resources and operation costs in recent years, recycling of the used toner material has been attracting considerable attention. For example, Japanese Laid-Open Patent Application No. 6-175488 discloses that the used toner is collected and returned by a toner transport unit to a developing unit to be admixed with fresh toner material, and is subsequently reused for forming toner images on the image bearing member.
As to the above noted toner transport unit, a powder screw pump (or powder pump) is generally included for its design flexibility and capability of transporting powder materials. In addition, the process of the toner transport is also known, which is pneumatically carried out for an admixed system of the recovered toner and gaseous flow generated by an air pump (Japanese Laid-Open Patent Application No. 11-73079).
The powder pump is formed, as previously known, to be a suction type uniaxial-eccentric screw pump (i.e., uniaxial pump having eccentric screw structure), including at least a stator which is provided with a through hole, and a rotor. The rotor has a screw-shaped surface structure, and is rotatably interfit to the stator along the axis of the cylindrical holder in contact with inner face of a wall of the stator. This uniaxial-eccentric powder pump is also known to be capable of transporting a relatively constant amount of material continuously at high mixing ratio with air, to thereby be able to attain precise amount of the toner transport.
An image forming apparatus has been previously disclosed in Japanese Laid-Open Patent Application No. 2000-47465 by the present inventors, in which the toner is transported by such uniaxial-eccentric powder pump incorporated into the apparatus.
In that disclosure, the powder pump in the image forming apparatus was designed, as shown in FIG. 7, to be driven directly by a motor 100 which also serves to drive major parts of the apparatus such as sheet supply unit, and developer and fixing stations. In addition, the developer station herein utilized a two-component developer containing toner and carrier components, and the station was controlled to maintain the toner concentration approximately constant by means of a toner supply unit. With this construction, the toner supply unit was able to support any mode of printing operation including solid monochrome printing.
The amount of toner transported in unit time by the powder pump was measured as a function of the number of rotor revolution of 125 rpm, 250 rpm and 400 rpm, with a pump having similar characteristics for identical distance and elevation to transport path. The results obtained from the measurements are shown in FIG. 8.
The results indicate, although the amount of toner transported in unit time reach approximately the same level of 50 mg/sec with the transport time of 500 msec or larger for each of rotor revolution of 125 rpm, 250 rpm and 400 rpm, the transient change during leading periods is considerably different (FIG. 8).
It should be noted herein that the number of revolutions for the main driving motor 100 may arbitrarily be adjusted, for example, to be smaller for relatively thick transfer sheets, or to be larger in a faster printing mode. When the number of revolutions decreases for the main motor, therefore, the number for the rotor also decreases accordingly.
As a result, for the previous construction of the toner transport system, difficulties may be encountered such that responsive toner supply can not be carried out for the slower mode of printing, to thereby not be able to produce a maximum sized printing in solid color.
Also, the distance and elevation of the transport path may be different. For example, for a full color image forming apparatus equipped with a toner supply unit incorporating plural powder pumps, the distances for the toner transport L1, L2, L3 and L4, and the elevations (differences in height) H1, H2, H3 and H4, for the stations 4M, 4C, 4Y and 4Bk, respectively, differ from each other as illustrated in FIG. 9.
As a result, the toner transport capability also differs for respective toner transport paths, thereby causing further complications for the toner transport including its control system, among others.
It is therefore an object of the present disclosure to provide a toner supply unit and image forming apparatus incorporating such supply unit, which are capable of implementing responsive supply of the toner unaffected by the operation mode change with preferably simpler construction of the supply unit.
In another aspect, with the decrease in the overall size of a development unit in recent years, there exists increasing needs of more precise control of the powder pump, which will be detailed herein below.
Utilizing the powder pump, the toner transporting unit is devised to be capable of securely transporting the toner, which is recovered from the cleaning unit, admixed with gaseous flow through a transport tube made of elastic materials.
From the consideration of durability, sealing performance, temperature increase, and powder scattering, the secure transportation can be achieved by operating the pump not continuously but intermittently over specified periods of time, when the amount of toner in a toner reservoir is measured, by a toner level detecting means, and then found to have reached a predetermined value.
In addition, also disclosed is the toner transporting unit incorporating the powder pump, which is alternatively configured to be capable of initiating the intermittent pump operation when an accumulated count of copied sheets reaches a predetermined number, in place of the above-mentioned method for determining the timing for initiating the pump operation based on the detected amount of toner in the toner reservoir.
The toner transporting unit utilizing the powder pump is thus operated, as noted above, not continuously but intermittently. This operation mode consists of, for example, driving with a unit driving time of 0.1 sec, 0.2 sec, etc. and halting with a unit halting time of 3 sec, 4 sec, etc. That is, the intermittent operation may be iterative cycles repeated a certain times, each cycle consisting of 0.1 sec of driving followed by 3 sec of halting, for example, thereby attaining a desired amount of the toner supplied to the developing unit.
With the decrease in the overall size of a developing unit in recent years, the amount of developer stored therein has also decreased.
Since the concentration of the toner in the developer tends to considerably fluctuate with more ease in the developing unit in such reduced size, the amount of toner supplied by the powder pump has to be controlled more precisely to maintain proper supply amount and concentration of the toner.
However, there has not been achieved so far methods for implementing precise control of the proper toner amount including proper consideration of the characteristics of powder pump.
These characteristics of powder pump are illustrated in FIG. 15, for example, which includes graphical plots illustrating the change of the toner amount (g) with time for unit driving times switched among the values of 0.1 sec, 0.2 sec and 0.3 sec, and with the number of rotor (rpm) kept constant.
Referring again to FIG. 15, after carrying out a first set of intermittent drives with unit driving time of 0.1 sec repeated 15 times, the unit driving time is switched to 0.2 sec, in which the desirable toner supply amount of 0.04 g for the unit driving time of 0.1 sec is approximately attained and remains stable at that level.
When the unit driving time is subsequently switched to 0.2 sec, the desirable toner supply amount of 0.08 g for the unit driving time of 0.2 sec can not be reached immediately after initiating a second set of intermittent drives with unit driving time of 0.2 sec. Furthermore, this desirable amount of 0.08 g is achieved only after a delay period which corresponds to 10 times of intermittent drives with the 0.2 sec unit time.
A similar delay period is also observed for the intermittent drive with the unit driving time switched from 0.2 sec to 0.3 sec. In a similar manner, when the unit driving time is switched from a one unit driving time (e.g., 0.2 sec) to a shorter one (0.1 sec), the toner supply amount can not decrease immediately, but the target amount of toner supply can be achieved only after another certain delay period.
As described herein above, when the unit driving time is switched from one to another in the previous methods utilizing the powder pump, the desirable supply amount of toner can be achieved only after delayed supply period corresponding the transient change (either increase or decrease) in the supply amount of the toner.
That is, immediately after switching the unit driving time, the change in toner supply amount can neither follow the switching speedily, nor attain the desired supply amount corresponding to newly adjusted unit driving time, but this desirable amount can be achieved only after a delay period, whereby another stable level of the amount of toner supply can be attained.
It is found through the above noted experimentation by the inventors that, when a set of intermittent drives are carried out following the previous set of intermittent drives with different unit driving time, a speedy change in supply amount can not be achieved possibly due to the toner left out in the powder pump during the previous intermittent drives, for example.
The present inventors further investigated the reasons for such a delay. The powder pump herein is devised of a stator which is fixed in the interior of a cylindrical holder, having a screw-shaped hollow structure inside thereof, and a rotor with another screw-shaped surface structure which is provided rotatively along the axis of the cylindrical holder in contact with inner face of the walls of the stator.
Since the rotor is provided with this structure to be enshrouded by the stator while retaining a passage formed along the cylindrical axis, the toner can be transport by the air pressure generated inside the passage by the rotation of the rotor.
In addition, the transport capability of the powder pump is therefore proportional to the cross-sectional area of the passage between the stator and rotor, and the amount of toner to be transported in unit time is that amount transported through the passage area in unit time.
If the size of the toner particles is quite small, it is found that the passage of the particles after switching the mode and conditions for the transport is affected by the properties (such as specific gravity and density) of toner which is left out during a previous transport mode.
Also found is that the effect of the previous transport mode on the following mode persists until the entire toner related to the previous drive is disposed completely from the inner space between the stator and rotor, and then a stable amount of toner supply can be achieved after this complete disposition.
It is therefore another object of the present disclosure to provide a method for implementing precise control of the toner transport by means of a powder pump incorporated in an image forming apparatus through suitable intermittent drives including proper consideration of the characteristics of powder pump.
To be more specific, the object is to provide the methods capable of alleviating undue delay in responding to the desirable change in the amount of transporting the toner by means of the powder pump, even when the unit driving time is changed.
In addition, it is another object to find a relationship, which can specify the number of repetitive transient drives carried out between one set of intermittent drives with a first unit driving time and another set of intermittent drives with a second unit driving time, and which is able to alleviate undue effects of the first unit time drives on the second unit time drives.
Accordingly, there are provided in the present disclosure a method and apparatus for implementing precise control of the proper amount of transporting toner by a powder pump incorporated in a copying apparatus, having most, if not all, of the advantages and features of similar employed methods, while eliminating many of their disadvantages.
The following brief description is a synopsis of only selected features and attributes of the present disclosure. A more complete description thereof is found below in the section entitled xe2x80x9cDescription of the Preferred Embodimentsxe2x80x9d
A toner supply unit is disclosed herein incorporated into an image forming apparatus for supplying toner from a toner container to a developing unit by a powder pump.
The powder pump includes at least a stator provided therein with a through hole and a rotor rotatably interfit to the through hole in the stator, and is characterized by being driven by its exclusive motor independently of a main motor provided for driving major parts of the image forming apparatus. This toner supply unit is further characterized by the number of revolution of the powder pump of equal to, or greater than 250 rpm.
According to another aspect, a toner supply system with plural powder pumps is disclosed incorporated into an image forming apparatus for supplying toner. The image forming apparatus includes at least a plurality of developing stations, and the toner is supplied to these developing stations from respective toner containers by respective powder pumps included in the toner supply system.
Each of the powder pumps includes at least a stator provided therein with a through hole and a rotor rotatably interfit to the through hole in the stator, and is characterized by being driven by an own exclusive motor independently of a main motor provided for driving major parts of the image forming apparatus.
This toner supply system is further characterized by the number of revolutions of each of the powder pumps adjusted either individually in advance or corresponding to the distance and elevation of toner transport path to each of the plurality of developing stations.
According to still another aspect, an image forming apparatus disclosed herein includes at least a developing unit for forming a toner image by developing a latent image formed on an image bearing member using toner supplied to the image bearing member, a powder pump for transporting the toner to the developing unit, and a control unit for driving the powder pump.
The powder pump herein includes at least a stator provided therein with a through hole, a rotor rotatably interfit to the through hole in the stator, and a motor for rotating the rotor, and is configured to transport toner by drawing from one end of the through hole, disposing through the other end of the through hole, and conveying the toner to the developing unit.
The powder pump is characterized by being driven intermittently with an arbitrary unit driving time, and, when the arbitrary unit driving time is changed to another unit driving time, driven transiently with still another unit driving time different from either the arbitrary unit driving time or the another unit driving time.
This image forming apparatus is further characterized, when the arbitrary unit driving time is changed to another unit driving time, by the least number, X, of driving the powder pump with the arbitrary unit driving time for causing undue delay of toner transport, is determined by the relation
Xxe2x89xa760P/nT, 
where p is a number of rotor pitch, n is the number of rotor revolution (rpm), and T is the arbitrary unit driving time.
The image forming apparatus is also characterized by the largest number, Y, for limiting the transient driving of the powder pump with still another unit driving time, which is carried out following the intermittent driving of the powder pump performed for X times with the arbitrary unit driving time, is determined by the relation
Yxe2x89xa760P/nT, 
where p is a number of rotor pitch, n is a number of rotor revolution (rpm), and T is the still another unit driving time.
According to another aspect, a method is disclosed for controlling an amount of toner transported by a toner supply system incorporated into an image forming apparatus for supplying toner.
The image forming apparatus includes at least a plurality of developing stations, and the toner is supplied to the plurality of developing stations from respective toner containers by respective powder pumps included in the toner supply system.
Each of the powder pumps includes at least a stator provided therein with a through hole, and a rotor rotatably interfit to the through hole in the stator.
The present method for controlling an amount of toner transported includes at least the step of driving each of the powder pumps by its own exclusive motor independently of a main motor provided for driving major parts of the image forming apparatus.
This method is characterized by the number of revolutions of each of the respective powder pumps adjusted either individually in advance or corresponding to the distance and elevation of toner transport path to each of the plurality of developing stations.
Further methods for controlling an amount of toner transported by a powder pump are also described in this disclosure.
The present disclosure and features and advantages thereof will be more readily apparent from the following detailed description and appended claims when taken with drawings.